Recent practices in oil exploration require that electric power be made available at the far end of an oil exploration well to power a variety of instruments that aid in navigating the drill head. Because the drill head could be up to five miles away from the surface, extendible power feeders have proven not only costly to operate, but also given to extremely high losses. Exploration companies and drill operators have therefore come to require that electric power be generated remotely at the end of the oil exploration well.
The typical remote power generation unit includes a turbine and an electrical generator which is coupled to the turbine via a transmission shaft. The turbine is positioned within the well in direct contact with the drilling fluid whereas the electrical generator is encased within a housing to isolate it from the drilling fluid. The turbine is caused to rotate by the drilling fluid flowing past it. The turbine imparts its rotational motion to the electrical generator via the transmission shaft. Specifically, the transmission shaft extends into the electrical generator housing to drive a rotor of the electrical generator. A seal is provided around the transmission shaft to prevent the ingress of drilling fluid into the generator housing.
The conventional remote power generation unit suffers from a number of disadvantages. A major disadvantage is that such units tend to fail prematurely. Premature failure results from leaks of drilling fluid into the generator housing. The drilling fluid, which is corrosive, causes the winding insulation in the armature to fail prematurely. Another disadvantage is the difficulty of scaling the housing effectively from the drilling fluid. The generators arc usually run in an oil bath to provide a degree of temperature and pressure control. Accordingly, the pressure of the oil bath must be regulated to equalize it with that of the drilling fluid. The difficulty is also increased by the fact that the typical electrical generator used in these applications comprises a conventional concentrically arranged generator. In a conventional concentrically arranged generator, a cylindrical air gap must be maintained between rotor and stator. A complex pressure equalization device must therefore be employed to regulate the pressure differential between the two liquid media.